1. Field of the Invention
The invention relates to a projection system, and especially relates to a touch projection system.
2. Description of the Prior Art
Recently, touch applications develop rapidly. Some kinds of projectors are designed to cooperate with touch technology. Common touch technology at present is to form an infrared light curtain in front of a screen. A receiving module is added into a projector for receiving image light produced by the infrared light curtain, so that a touch operation of a user can be determined by analyzing an image of the user interrupting the infrared light curtain. In practice, the receiving module can be integrated with a projection lens of the projector. The image light enters the projector through the projection lens and is reflected by a micromirror device (e.g. a digital micromirror device, DMD) to an image-capturing device. Furthermore, in some kinds of projectors, a total internal reflection (TIR) prism is disposed in front of the DMD. Projection light produced by a light source device is reflected by the TIR prism to the micromirror device, is reflected by the micromirror device to be projected onto the screen to form a projection image. In this configuration, the image light from the screen passes through the TIR prism, is then reflected by the DMD, passes through the TIR prism again, and is received by the image-capturing device. After the image light is reflected by the DMD, an optical distance for the image light to travel in the TIR prism varies as traveling direction, which leads to a distortion of a touch image formed by the image light received by the image-capturing device so that the corresponding touch operation may be determined incorrectly or fail.